1. Field of the Invention
The present invention relates to a strength evaluation method for brittle material pieces, in particular, a strength evaluation method for brittle material pieces which can accurately evaluate the strength of brittle material pieces such as ceramics and glass while, restraining pieces that passed a test from undergoing strength degradation, and eliminate pieces having low strength.
2. Discussion of Background
When brittle material pieces (including a product made from a brittle material) such as ceramics and glass are used under a high stress, a proof test has been carried out prior to use to see whether the brittle material pieces are fractured by application of a predetermined stress. In the proof test, brittle material pieces which have not been fractured are determined as survived pieces, and low strength pieces which have been fractured are eliminated as pieces that failed the test. However, because the stress is applied to the brittle material pieces in the proof test, the survived pieces which have not been fractured by the proof test include ones which have undergone strength degradation caused by accumulation of damage due to e.g. crack growth. To what extent such strength degradation causes varies depends on not only the kind of the brittle material but also the magnitude of the applied stress.
In general, it is considered that no plastic deformation causes in brittle materials, accumulation of damage at a crack tip is relatively small and no fracture is caused by gradually accumulating damage as a result of application of a stress, and that brittle materials have linear fracture behavior wherein elastic deformation continues until a certain limit stress is exerted, and immediate fracture causes at the instant when a stress exceeds the limit.
Since brittle materials have the linear fracture behavior, accumulation of damage in the survived pieces is relatively small and strength degradation is not susceptible in the proof test when a stress applied to a brittle material piece is fairly smaller than the material strength of the brittle material piece. However, when a high stress close to the material strength of the brittle material piece is applied, strength degradation is susceptible to cause.
For those reasons, a conventional strength evaluation method for brittle material pieces has eliminated only significantly low strength pieces as pieces that failed the test by carrying out a so-called proof test wherein a stress which is fairly lower than the average strength of a group of brittle material pieces is applied for a predetermined period of time and is released.
However, a product which is made from a brittle material is in some applications required to be used under several tens to over one hundred percent of the average strength. In such cases, it is necessary to evaluate the strength by applying to the product a stress close to the average strength. In the conventional strength evaluation method, the ratio of brittle material pieces which have fractured in the proof test increases, and that many pieces which have caused strength degradation are existing in pieces determined as the survived pieces.
Because most of conventional brittle material pieces which have been subjected to such a proof test are dense and substantially homogeneous structural material (e.g. mechanical parts), and have such outstanding linear fracture behavior, the conventional proof test is applicable. However, the linear fracture behavior is not applicable to a heterogeneous brittle material such as porous ceramics used in a filter of a filter apparatus, and a ceramics composite material with particles and fibers distributed in a ceramic matrix. Application of a stress in the test causes strength degradation in them, and even application of a smaller stress than the average strength causes strength degradation due to accumulation of damage. The conventional strength evaluation method has the problem in that a product which has caused strength degradation is existing in products determined as survived pieces.
This problem is significantly important because for example if only one filter tube is broken for some reason in a filter apparatus for a hot gas with a large number of porous ceramic filter tubes mounted thereto, a large system such as a power plant shuts down in the entirety.